1. Field of the Invention
This invention relates generally to leak detection and specifically to detection of methane gas, particularly for use in conducting routine surveys of natural gas utility pipelines.
2. Description of the Prior Art
Natural gas utility companies operate approximately 61 million customer gas meters in the United States alone, supplied by over one million miles of pipeline, 17 hundred transmission stations and 17 thousand compressors. To maintain security and integrity of this vast distribution system and to comply with regulatory requirements, gas pipelines are subject to regular inspections to detect leaks. Routine periodic leak surveys are typically accomplished by a walking survey, a mobile survey or an aerial survey, and it is estimated that the natural gas distribution and transmission industry spends over $300 million annually to survey the pipeline network for leaks. Because varying trace levels of methane, ethane, propane and butane are found in the atmosphere from decaying organic material, flatulence, et cetera, survey instruments must be capable of distinguishing naturally occurring background gas levels from levels indicative of a natural gas pipeline leak.
Surveys are generally performed using one or more detectors relying on varying detection methods and principles. For example, combustible gas indicators (CGIs), which sense all combustible gases, may be used for walking surveys. CGIs work on the principle of catalytic combustion of a gas sample. They are generally unable to detect gas mixtures much below the lower combustible concentration limit, and thus they have generally low sensitivity and do not readily detect low gas concentrations. Additionally, the sample probe must be located within the gas plume in order for the CGI to obtain a reading. Therefore, CGIs are generally suitable only for walking surveys where the operator can move the probe along the entire length of the gas line.
A more sensitive leak detector commonly used is the flame ionization detector (FID). The FID operates on the principle of measuring electrical conductivity of a flame burning carbon compounds. Like the CGIs, FIDs sense all combustible gases and require the sampling probe to be placed within the gas plume in order to detect the gas leak. However, while the CGI typically measures gas concentration in percentage, the FID typically measures gas concentration in parts per million (ppm). The FID is useful for walking, mobile and airborne surveys, but only by traveling through the leak plume.
The CGI and FID both typically use an extractive sample or measurement path. In this method, target gas concentration is measured by a detector installed in a measurement chamber through which gases of interest are continually drawn from the immediate surrounding atmosphere via the probe. There is generally a detection delay of a few seconds associated with the time required to draw the gas sample into the measurement chamber.
An optical methane detector (OMD) operates by absorption of infrared light by methane. Because natural gas primarily contains methane gas, detection of methane gas serves for detection of natural gas. It is well known that gas molecules absorb energy in narrow bands surrounding specific wavelengths in the electromagnetic frequency spectrum. For example, methane has strong absorption bands at 1.33 μm, 1.67 μm, 3.3 μm, and 7.6 μm. At wavelengths falling even slightly outside the narrow absorption band, there is essentially no absorption. Thus, the OMD measures the attenuation of an infrared light source passing through a gas sample at the methane-characteristic absorption wavelength to determine the presence of methane gas. Therefore, the OMD is more selective than either the CGI or the FID, because it measures methane specifically and not all combustible gases. The OMD generally uses a short open path sample method which eliminates the sampling time delay associated with extractive sampling method of CGIs or FIDs. In a short open path configuration, the light source is transmitted across a line of sight and is either reflected to an optical detector by a fixed reflector of known characteristics located only a short distance from the light source, or the light is received directly by a fixed detector located only a short distance away from the light source. The OMD sensitivity in detecting methane is of the same order of magnitude as the FID. However, the short open probe must still be immersed in the leak plume for detection to occur. Therefore, like the FID, the OMD is useful for walking, mobile or airborne surveys only by traveling through the leak plume.
During a walking survey, a person walks above the service line and uses an instrument to detect gas escaping from the pipe, meters, fittings, etc. Because the commonly available gas detectors such as CGIs, FIDs or OMDs must be positioned within the leak plume to detect the presence of methane, a service person must usually walk the entire length of the service line between the main and the customer gas meter to ensure that low level gas emissions are not missed. The procedure may be arduous, particularly when access to private property is involved. The walking surveys not only require the surveyor to know fairly accurately where the gas pipe is under the ground, but may also require access to residential or business property to properly survey the entire system. Obstacles such as fences and dogs can be problematic.
In a mobile survey, a vehicle is driven as close to the pipe as possible while the vehicle remains on the right-of-way. FIDs or OMDs are used to detect gas escaping from the pipe, meters, fittings, etc. and collecting in plumes which cross the right-of-way. Unless the leak is located close to the right-of-way or a fortuitous breeze exists, the plume must generally be large in order to reach the right of way and hence is likely to be of lower concentration and more difficult to detect.
For an aerial survey, an aircraft flies at a low altitude along the right-of-way using visual, flame ionization or infrared techniques to identify major sources of gas leakage along the route of the gas pipe. Aerial surveys in general are used to identify larger emissions. These larger emissions may cause major vegetation discoloration or disruptions to the pipe cover. Aerial surveys with gas detection instruments rely on the fact that natural gas is lighter than air and will rise into the atmosphere where it may possibly be detected by an instrument mounted on the aircraft.
Both the flame ionization and infrared detector instruments require that readings be made near the source of the emission or that a representative sample of the atmosphere above the gas pipe be received within a sensor for indication. However, a recently developed technology, referred to herein as a laser methane detector, which uses wavelength-modulated laser absorption spectroscopy, specifically tunable diode laser absorption spectroscopy (TDLAS), may greatly simplify and economize walking and mobile surveys. Referring to FIG. 1, laser methane detectors (10) containing a tunable diode laser, an optical detector capable of detecting the reflected light emitted from the laser, and associated detection circuitry, can be used by a technician (12) to survey a gas line installation (14) from a distance. Laser methane detectors significantly reduce the need for the surveyor to gain access to private property (16) and walk the entire length of the service pipe (14) to complete the survey, thus realizing an estimated productivity improvement of between twenty and forty percent.
TDLAS gas analyzers rely on well-known spectroscopic principles and sensitive detection techniques coupled with advanced diode lasers. Gas molecules absorb energy in narrow bands surrounding specific wavelengths (sometimes referred to as absorption lines) in the electromagnetic spectrum. At wavelengths slightly different than the narrow absorption bands, there is essentially no absorption. Specifically, when the laser wavelength is tuned to correspond to a particular absorption band of a target gas molecule, the light transmitted across a measurement path containing the gas is attenuated according to the Lambert-Beer relation,Iν=Iν,o e−S(T)g(ν−νo)Nl)  (1)
where:Ivis the received intensity;Iv,ois the initial laser intensity;vis the laser frequency;lis the optical path length through the gas;S(T)is the temperature-dependent absorption linestrength;Nis the target species number density; andg(v − vo)is the lineshape function which describes thefrequency dependence of the absorption linestrength.The argument of the exponential function is the fractional change in the laser intensity across the measurement path and is conventionally known as the absorbance. By transmitting a beam of light (18) through a gas mixture sample containing a quantity of the target gas (20), tuning the beam's wavelength to one of the target gas's absorption bands, and accurately measuring the absorption of that beam, the concentration of target gas molecules integrated over the beam's path length, l, can be accurately determined.
Until the 1990s, TDLAS was suitable only for laboratory-based gas analysis, because it required highly-skilled individuals to operate and maintain the complex devices and provide expert interpretation of the results. During the past decade, however, the technology has developed primarily because of the advent of reliable monochromatic near-infrared (1.2–2.5 μm) diode lasers that operate continuously and unattended at room temperature without cooling by liquid nitrogen. These lasers, particularly the distributed feedback variety with grating-like optical elements which force each laser to emit light at a specified wavelength, offer line widths less than 0.003 cm−1, which is considerably narrower than molecular absorption line widths (typically 0.1 cm−1 at atmospheric conditions). By accurately controlling the laser temperature and current, the laser wavelength may be rapidly and precisely tuned over a range of about ±2 nm around its specified wavelength. Similarly, vertical cavity surface emitting lasers provide suitable performance characteristics in the 700–900 nm wavelength range.
With laser methane detectors (Applicant's assignee, Heath Consultants, uses a trademark, RMLD, to designate its version of a laser methane detector), a tunable diode laser beam (18) is transmitted onto a distant (e.g., up to 100 feet) topographic target (22). Some of the laser light (24) is reflected by the target back to an optical detector co-located with the laser in the laser methane detector in what is referred to as a stand-off measurement path. The laser has a specific design wavelength chosen to optimize the sensitivity to methane gas (e.g., 1.6537 μm, a wavelength corresponding to an absorption line of methane which is also free of interfering absorption from other molecules). The laser's fast tuning capability is exploited to rapidly and repeatedly scan the wavelength across the gas absorption line. While this scanning occurs, the fraction of emitted laser power that is transmitted through the gas mixture and reflected back to the instrument is received and measured by the optical detector. When the wavelength is tuned outside of the narrow characteristic absorption band (“off-line”), the received light is equal to or greater than when it falls within the narrow absorption band (“on-line”). Measurement of the relative amplitudes of off-line to on-line reception yields a precise and highly sensitive measure of the concentration of the methane gas along the path transited by the laser beam. The collected light is converted to an electrical signal, which is processed so that methane column density (the methane concentration integrated over the beam length) can be reported, usually in ppm·m. Typically, the laser methane detector rapidly processes discreet measurements at a refresh rate, e.g., of 10 Hz.
Referring to FIG. 2, an laser methane detector (10) generally includes a number of functionally interactive components: a laser emitter subsystem (30) that contains a laser source module and electronic modules that synthesize the laser modulation and control signals; an optical detector (32), e.g., a photodiode; a signal processing module (34) which contains the electronic components that extract the absorption signal information from the optical detector's output; a system controller (36) (usually microprocessor-based); and a user interface module (38), e.g., a LCD alphanumeric display or audio output. The laser beam transmitter and receiver may be placed in a combined optical assembly (not shown). laser methane detectors (10) are typically packaged into a hand-held gun (40) including an optical transceiver and an alphanumeric display and a shoulder or waist-mounted unit (42) with some controller circuitry (36) and a rechargeable battery pack (37). The two sections (40, 42) are generally connected by an umbilical cable (44) with optical fibers and electrical wires.
Some laser methane detectors currently employ an audio tone user interface in order to aid the user in identifying gas leaks from natural background methane levels. As the detected methane level increases, the output tone linearly increases in pitch, for example, according to the equation
                    T        =                  {                                                                                          c                    ·                    M                                    =                                      c                    ·                                          k                      ⁡                                              (                                                                              f                            2                                                    /                                                      f                            1                                                                          )                                                                                                                                                              for                    ⁢                                                                                  ⁢                                          f                      1                                                        ≥                                      10                    ⁢                                                                                  ⁢                    or                    ⁢                                                                                  ⁢                                          f                      2                                                        ≥                  0.5                                                                                    0                                                                                  for                    ⁢                                                                                  ⁢                                          f                      1                                                        <                                      10                    ⁢                                                                                  ⁢                    and                    ⁢                                                                                  ⁢                                          f                      2                                                        <                  0.5                                                                                        (        2        )            
where:Tis the output tone level (Hz);Mis the calculated methane column density (ppm · m);cis a tone coefficient, typically 10 Hz/ppm · m;f1is the off-line reflection intensity;f2is the on-line reflection intensity; andkis a conversion constant.
If both the off-line and on-line return intensities are too low, the laser methane detector outputs no tone to indicate a low-light condition. Otherwise, the user may become crazed by a continuous cannonade of cacophonous tones. The frequency of the tone changes from sample to sample, causing the user to perceive noise. The user of the instrument must then be able to distinguish tone pitch changes that are indicative of a gas leak. Because the tone level increases with the detected methane, high gas concentrations are generally easy for an operator to recognize, but more subtle pitch changes indicative of smaller leaks may go unnoticed. Because the user is forced to carefully listen to discordant tones throughout the entire survey, surveying can be particularly fatiguing for the user.
Additionally, there are several characteristics of stand-off detection that hamper the user's ability to distinguish background methane from methane resulting from a natural gas leak. First, even with constant natural background methane levels, as the scanning distance increases, the methane column density (methane concentration—beam length product) measured by the laser methane detector increases, causing the output tone pitch to rise. In other words, as the scan distance increases, the laser beam passes through more natural methane background, and the tone pitch increases. Second, the variance in the reflected light levels at the laser methane detector increases as the scanning distance increases causing the tone pitch to more rapidly change up and down. This phenomenon may be due in part to increased deflection and backscatter of both off-line and on-line signals as the light passes through more atmosphere. Third, changes in the reflectance of the various topographical surfaces increases the variance in the received light levels at the laser methane detector. For example, when the scan crosses a sharp transition between two different surface types, such as an asphalt-grass interface, a high pitch tone may be heard even though there is no gas leak.
Further, other difficulties hamper detection of gas leaks. First, beam power, size, and calibration of the instrument all affect the detection effectiveness. Second, with too low a signal/tonal update rate, the user may miss a very short “hit” of one or two samples. Third, the digital display is of limited assistance in finding small leaks due to a slow screen update rate (typically 3 Hz), and the fact that it is difficult to focus simultaneously both on where the laser beam is being directed and the digital display.
Because of these limitations and characteristics, most users find it difficult to discern a leak by tonal difference when there is a high background signal or for low level leaks. The ability of a user to discern a subtle tonal change when the background is high and when there is a high variance, e.g. during long scanning distances, is very difficult if not impossible. The ability to discriminate leaks based on tone change is also dependent on the user's hearing ability, and experience in listening to the tonal response under different conditions. In addition, differences in sound from unit to unit may make it more difficult for a user to switch between units with equal results.
A laser methane detector having sophisticated data processing capabilities which take into account signature and signal characteristics to enhance a user's ability to detect a gas leak over reliance simply on tones generated from raw methane column density data is desirable.
3. Identification of Objects and Features of the Invention
An object of the invention is to provide a method for optical leak detection and an optical detector therefor that uses statistical data analysis in real-time or near-real-time to aid the user while surveying in the field to differentiate gas levels greater than the general background levels, and possibly indicative of a leak or of interest, from background gas levels.
Another object of the invention is to provide a method for optical leak detection and an optical detector therefor which does not need to be located within the gas leak plume in order to detect a leak and which is suitable for walking surveys.
Another object of the invention is to provide a method for optical leak detection and an optical detector therefor which promotes user comfort, enhances leak identification and reduces user fatigue by outputting audio tones for detected gas levels of interest and silence at lower gas levels.
Another object of the invention is to provide a method for optical leak detection and an optical detector therefor which can detect methane plumes with a concentration as low as 5 ppm-m and as far away as 100 ft.
Another object of the invention is to provide a method for optical leak detection and an optical detector therefor which is sensitive under varying weather conditions.
Another object of the invention is to provide a method for optical leak detection and an optical detector therefor which may be handheld.